Mr. Drach on the Horary Deviations of the Barometer. 477 



Again, when x is given we can find n from the equation 



a^— 3^ + 2(1— «) = 0, 



2 + sc^—Sx 

 or n = . 



2 



Thus let the axes of the two eUipsoids be in the ratio of 

 2 : \, X = \ and 7i = ■^■^, or a plane which cuts off -^^ of an 

 ellipsoid envelopes another whose axes are one half those of 

 the given one. 



Hence also the theory of floating bodies, whose surfaces are 

 of the second order, may be reduced to the comparatively 

 simple problem, of similar and similarly situated bodies, rest- 

 ing in equilibrio on a horizontal plane. 



LXX. On the Horary Deviations of the Barometer as ob- 

 served at Plymouth. By S. M. Drach, Esq., F.R.A.S. 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 

 TN the Report of the British Association for 1839, the de- 

 -*- viations of each hour from the line of mean pressure 

 (= 29'7999 inch.) is given in Tab. xix. p. 164. Taking the 

 unit for the avoidance of decimals to be the ten-thousandth of 

 an inch, the deviation at 



A.M. 



1 h' = +18 



- 6 

 -55 

 -71 

 -71 

 = -39 



A.M. 



7h' 



8 



9 

 10 

 11 

 12 



= + 3 

 + 33* 

 + 49 

 + 62 

 + 46 



= + 3 



P.M. 

 1 h'-=- 



42 



— 77 



— 91 

 -104 



— 61 



:— 29 



P.M. 



7h'-=+ 20 

 8 +62 



+ 95 

 + 100 

 + 93 

 = +66 



9 

 10 

 11 

 12 



Adding together hours of the same name (as 1 a.m. 

 1 p.m.), there results, 



1 hr. = — 24 7 hr. = 



and 



8 



9 



10 



11 



12 



+ 24 

 + 95 

 + 144 

 + 162 

 + 139 

 + 69 



Hence, from the hours 1 and 7, 3 and 9, 6 and 12, it might 

 be inferred that the law of these aggregate oscillations is 

 nearly representable by the formula B sin 2 ^ + i cos 2 1 ; 

 where B, b are constant for one day, and t — the time from 

 midnight, supposing one day = 360°. 



• In:«tead of 32, to iigree with Tab. I. p. 150. 



